1. Field of the Invention
This invention relates in general to footbridges and in particular to a footbridge formed of fiberglass.
2. Description of the Prior Art
There is a need for corrosion resistant footbridges and walkways in locations with highly corrosive atmospheres. For example in waste water treatment facilities, large tanks or ponds containing water being treated require footbridges to reach pumping equipment located in the center of the ponds. These footbridges may be as long as 70 feet or more, and are normally suspended only at the ends. They are normally constructed of steel, and even though coated with a protective coating, may last only six years because of the highly corrosive environment.
Fiberglass structures are used for many purposes and are known to be resistant to corrosion. One limitation on their use for bridges results from the modulus of electricity for fiberglass, which is much lower than the modulus of electricity of steel. One may increase the rigidity of a fiberglass member by putting more fibers in it, but if one wishes to reach the rigidity of steel member, the expense may become prohibitive. Fiberglass footbridges, however, are known to applicant. These footbridges are constructed of individual members bolted or bolted and bonded together to resemble a steel bridge. In addition to the high expense, these bridges are not entirely satisfactory in performance because composites have poor resistance to bearing and also crush under the bolt causing premature failure in some cases.
Consequently, to be competitive with steel footbridges, a fiberglass footbridge should contain structure that provides strength and rigidity without requiring individual structural members of high modulus of electricity. It is known that continuous parallel filaments of fiberglass formed in a member along its length will increase the bending strength and stiffness of the member. The use of continuous filaments is shown in U.S. Pat. No. 4,029,172 entitled Fiberglass Ladder filed July 12, 1976, which discloses a fiberglass ladder, and in U.S. Pat. No. 3,328,818, which discloses a ramp for unloading trucks. The ladder is for use in vertical or near vertical conditions, thus not subject in the same degree to bending and rotating forces as a bridge. The ramp does not have either a high span to depth ratio or a high width to span ratio, thus is also not subject to the same degree of bending and rotating forces.